Fabrication of CuInS2 films from electrodeposited Cu/In bilayers: effects of preheat treatment on their structural, photoelectrochemical and solar cell properties†
Polycrystalline CuInS2 films were fabricated by sulfurization of electrodeposited Cu and In metallic precursor films in a Cu-rich composition at 520 °C in H2S (5% in Ar). Structural analyses revealed that the adherence of the thus-formed CuInS2 film to the Mo substrate was strongly dependent on heating profiles of the Cu/In bilayer film: a CuInS2 film with poor adherence having many crevices was formed when the Cu/In bilayer film was heated monotonously from room temperature to 520 °C in Ar within 25 min followed by sulfurization, whereas CuInS2 films with good adherence were obtained when the Cu/In films were pretreated at 110 °C in Ar for 10–60 min just before increasing the temperature up to 520 °C for sulfurization. It was also clarified that the CuInS2 film obtained without 110 °C pretreatment had pinholes inside the film, whereas the CuInS2 films formed after 110 °C pretreatment showed no notable pinholes. Photoelectrochemical responses of these CuInS2 films in an electrolyte solution containing Eu(III) indicated that the CuInS2 films obtained after 110 °C pretreatment had higher external quantum efficiency (EQE) values than those of films obtained without 110 °C pretreatment, mainly due to better adherence of 110 °C pretreated CuInS2 films to the Mo substrate than the CuInS2 film obtained without 110 °C pretreatment. The performance of solar cells with an Al:ZnO/Zn(S,O)/CdS/CuInS2/Mo structure also depended on the structural characteristics of the CuInS2 films, i.e., preliminary conversion efficiencies of ca. 5% were obtained for devices based on the CuInS2 films obtained after 110 °C pretreatment, whereas the device prepared by the CuInS2 film without 110 °C pretreatment showed the conversion efficiency less than 1.5%.